A gate valve is a type of fluid-control valve that opens by lifting a round or rectangular gate or wedge out of the path of the fluid. A feature common to all types of gate valves is that the sealing surfaces between the gate and seats are planar. The gate faces can form a wedge shape or they can be parallel. Gate valves are sometimes used for regulating flow, but many are not suited for that purpose, having been designed to be fully opened or closed. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss.
Gate valves are characterized as having either a rising or a nonrising stem. Rising stems provide a visual indication of valve position. Nonrising stems are used where vertical space is limited or underground.
The bonnet provides leak-proof closure for the valve body. A bolted bonnet is used for larger valves and higher pressure applications.
Gate valves normally have flanged ends which are drilled according to pipeline compatible flange dimensional standards. Cast iron, cast carbon steel, gun metal, stainless steel, alloy steels, and forged steels are different materials from which gate valves are constructed.
A well-known draw back to gate valves is that in large-size, high pressure applications (defined herein as applications where the valve is greater than four inches in size and differential pressure on opposite sides of the gate is in excess of 5,000 psi), there is so much force on the gate that it is hard or impossible to open due to friction between the gate and its seal. Even where the gate is openable under such pressure, the friction between the gate and its seal, the force required on the actuator, and the blasting action of the initial equalization flow across the cracked gate, singly or combined may be highly destructive to the valve.
The conventional solution to the problem is an external equalizing conduit between the two sides of the valve, with a small needle valve or the like manually-openable to equalize the pressures on the gate prior to it being opened. This solution, however, is highly dangerous to the operator due to a tendency of the needle valves to violently fail when operated under pressure. Thus there exists a need for a safe and effective means to equalize the pressures on opposite sides of a gate valve in high pressure applications.